If you are the type of person who keeps one eye open while washing your face (the better to catch ghostly reflections in the mirror, of course), here's another horrifying bathroom scenario you might not want to hear: every time you turn on your shower, you are sprayed with a heavy mist of millions of bacteria, which get inhaled deep into your lungs.

One such microbe called Mycobacterium avium is similar to the bug that causes tuberculosis (TB) and causes lung infection. It is also found commonly in showers in New York and Colorado, according to a new study led by University of Colorado microbiologist Norman Pace, who studies bacteria found in homes, schools, public buildings and other human environments. (See the top 10 scientific discoveries of 2008.)

In this latest study, published in the Proceedings of the National Academy of Sciences, Pace dispersed a team of undergraduate students into 45 bathrooms mostly in New York City and Denver to swab and test the inside of showerheads; bacteria tend to accumulate in dense communities there, forming thin, gooey "biofilms." When you run the shower, germs are ejected out of the showerhead in the spray. Inhale the fine water droplets and M. avium gets a direct passage to your lungs where it proceeds to wreak havoc if your immune system isn't strong enough.

So, does this mean you should turn off the shower and go back to old-fashioned baths? Not exactly. "If you are an otherwise healthy person, there is no cause for alarm," says Dr. Lynn Connolly, a practicing clinician and assistant professor at the University of California, San Francisco, who has treated patients with M. avium infection. Connolly is quick to add that if you have AIDS, chronic lung disease, cystic fibrosis or an immune system disorder, then there is some cause for concern. In such patients, the bacteria can cause lung diseases, and in some extreme cases infections in other parts of the body as well. (See the most common hospital mishaps.)

Overall, M. avium-related disease is rare. The illness causes occasional fevers, a persistent cough and a general feeling of exhaustion that can last for months. Researchers estimate that two in every 100,000 Americans become sick from M. avium. That's about as many people in the U.S. with TB, but unlike TB, M. avium-related disease doesn't spread from person to person.

It's a good thing the bug doesn't do much harm to the average human, since it's everywhere  in the air we breathe, the water we drink, even in soil outside our homes. If you receive municipal water, then you're getting about 10 million of these and other microbes per liter of tap water, says Pace. And while it's possible that some people's disease may be specifically related to the bacteria that comes from the shower, the only way to know for sure is to genetically match the pathogen in infected patients with the bugs in their showers. A few such studies have been done in the past, but each time with a tiny sample of one or two patients.

For now, to avoid a face full of bacteria each morning (Pace's team identified more than a dozen microbes other than M. avium in the showers they tested), let the shower run for a while before getting in. And remember to keep one eye open for zombies.